Legal Question in Real Estate Law in California

You have a right to cut it back so long as it does not damage the tree's viability, but it is at your expense.

Overhanging tree limbs and invasive roots are trespasses at the point they cross the property line. A special set of rules have evolved from court decisions regarding tree trespasses. In addition to going to court and suing the neighbor for trespass, in which case your judgment would be that the neighbor must prune the tree to eliminate the trespass at his own expense, you have the option of self-help without the time and expense of going to court. However, if you elect the self-help remedy, you have two burdens: (1) you bear the cost of the trimming, and (2) you must do the trimming (or have it done) "in a non-negligent manner," to quote one court of appeal decision. This is generally construed to mean trimmed in a way that (a) the tree doesn't die, and (b) the tree is not made a danger, e.g., likely to topple over in a windstorm.